Question (Part 1):
Christianity seems to have been the first world religion that was interested in and successful with establishing itself as not only the state religion of but practically the only in an empire - namely, the late Roman Empire as its client and successor states.
I don't think Christianity was really "first" at any of those things. They weren't the first to persecute nor were they the first to be recognized as the official faith of a great empire.
Intolerance of other religions wasn't unique to fourth and fifth century Christians. The Christian experience in the first, second, third centuries at the hands of Judaism, and various Pagan religions is example enough to refute that.
Likewise Zoroastrianism was the state religion of the Persian Empire which was one of the greatest empires of it's time. Buddhism was a state religion of the Mauryan empire (322–180 BCE). Hinduism was the official religion of the Gupta Empire(c. 320-650 CE). All of which predate Nicene Christianity becoming the official religion of Rome in 380 AD.
Christianity broadly could be argued to be the last religion to obtain supremacy in Rome, but not the first. Likewise it's only with modern eyes that one can make this claim because the early church faced so many problems with heretics, schisms, and violent reformations that it's very hard to claim a monolithic "Christian" identity between cities which were still struggling with Christian unified doctrine nearly up until the time the western Roman empire blinked out of existence in AD 476.
Question (Part 2):
As I understand the timeline,
- Christianity was legalized by Constantine in 313 CE;
In Feb 313, Constantine the great(western Augustus) and his primary rival Licinius(eastern Augustus) agreed to peace terms including both signing the jointly authored Edict of Milan which granted religious freedom within both halves of the Roman Empire, yes this included tolerance of Christianity. However shortly after this agreement Licinius reneged on this pledge and once again returned to the policy of suppressing Christians. Christianity did not get lasting tolerance (* of a sort) until after Constantine become the sole Emperor of Rome after the Battle of Chrysopolis(September 18, 324). A war fought against Licinius for soul supremacy over Rome.
(*) There was no uniform Christian faith prior to the first Church Council held at Nicea(325 AD). Christianity had endured centuries of persecution and was organized into individual franchises with 1 all powerful bishop per City. These Bishops didn't communicate widely during the times of persecution, due to the risks of discovery and doctrinally had grown apart. They didn't agree on what it meant to be Christian. The Romans were all about Unity and the Pagan Emperor Constantine the Great called the first church council of all the Bishops to create a profession of faith and promote the desired Unity. Constantine called the Bishops to come to him, Nicea is a suburb of Constantine's eastern Roman Capitol Constantinople. Constantine didn't get all the Christian Bishop to attend, most of the invited Christian Bishops did not attend. Important Bishops like the Bishop of Rome(Pope) were absent, and those he did get disagreed. Under Constantine's leadership and political pressure the Bishops in attendance crafted a creed, the Nicean Creed which defined what it was to be christian. This Creed had the majority of the support of the Bishops in attendance at Nicea, but most of the Christian Bishops were not in attendance. So many powerful church leaders disagreed with it, by the time of Constantine's death it was already out of favor. After Nicea their wasn't consensus but at least after Nicea christians began separating themselves/organizing into different belief systems. (One Rotating officially recognized, the rest persecuted heretics) Whichever belief system had the emperor's ear became the legitimate faith, and the others were labeled heretical and there followers persecuted. The next three Roman Emperors after Constantine would not be Nicean Christians. The Nicean Christians which dominate today's Christianity (belief in the Trinity, and Jesus as an equal god head with the Father and Holy Ghost) were themselves heretical for several decades after the death of Constantine, who was attended on his deathbed (May 22, 337 AD) by an excommunicated Arian bishop (Eusebius of Caesarea) . Nicene Christians didn't regain control of the faith (Catholic Church) until 380AD when they were again recognized by the Emperor as the favored branch of Christianity.
Question (Part 3):
- pagan rituals were prohibited by Constantius II in or before 353 CE and again by Valentinian II and Theodosius.
Not all Pagan rituals, primarily those involving sacrifices.
Auguries, where a pagan priest would interpret the desires of the gods by the entrails of animals was outlawed under Constantine the Great and Constantius continued that policy along with the condemnation of magicians and fortune tellers who were kind of seen as con-men preying on the people.
Pagan religions continued in Rome long after Constantius II.
Constantius II himself was deified by the Roman Senate after his death.
Constantius II was actually fairly tolerant of Roman's Pagans. Again, Religion was pursued by the Roman Emperors to unify the empire, and that did not include persecutions of the majority of your citizens who were still Pagans under Constantius. Well unless the persecutions lead to consolidation of beliefs, then it was fair game. Constantius wasn't above persecution of Christians who didn't get with his doctrinal program, namely elevating Nicean beliefs above those of other Heretical teachings. Constantius is best remembered for trying to find a moderate path between two of the largest Christian sects which were vying for control of the Church. Those which follow the Nicean Creed which most Christians today subscribe too and a sect called Arianism which taught God as the Father predates Jesus the son. The difference between made not begotten, and begotten not made professed by most Christians in church every Sunday to this day in the Nicean Creed.
Question (Part 4):
- Finally, Theodosius, apparently pushed to it by Saint Ambrose seems to have led a violent campaign against paganism 392-395 CE (at a time when according to wikipedia still half the empire was pagan).
Theodosius became Emperor in the Eastern Empire in January 379AD. He became Emperor in the west in May of 392 AD, and he died in January of 395AD. During his turbulent 16 year rule he was at war for 15 years. 3 years with the Goths and 11 years with Roman rulers in the West. Theodosius crackdown on Roman's Pagans had everything to do with suppressing revolutionaries and rivals to his throne and less to do with Christianity.
Question (Part 5):
How did Christians manage to stamp out all other religions?
In the 4th Century it was mostly all about Rome and Rome's interest and pursuit of a unifying force in their fractured and still splintering empire.
In the first century AD the Roman Emperor Augustus Ruled for 45 years (31 BCE–14 CE). In the Second Century Rome had 9 Emperors. From the beginning of the third century 200AD to 306AD there had been 34 emperors. In 306 AD the life expectancy of a Roman Emperor could be counted on one hand(about 3 years on average). Constantine's father (Constantius I) had ruled as Augustus in the Western Empire for less than 1 year before dying in Britain during a war with the Picts. In 306Ad when Constantius I died, leaving Constantine as heir to the throne, Constantine obvious first thought was, how am I going to survive this. One of his first steps was to turned from persecutor of christians to advocate for the religion. His armies while made up almost entirely of Pagans (mostly Mithraism/Sun Cult ) fought under the sign of the cross. ( Christians were pacifists in 306AD ). Why did Constantine turn to Christianity? It was a strategy to unify and stabilize the Roman Empire. His life literally depended on finding a better way. To be named Emperor of Rome in 306 AD was otherwise a death sentence. He would promote the religion and influence it to yield the kinds of citizens he he believed christianity could help him grow.
I would argue the Roman Empire's religious persecution during the early decades of the Catholic Church, 4th Century, had more to do with state policy and politics within Rome than it did religious observance. The Roman Emperors of this period were on a sinking ship and they knew it.
They were looking for Unity and Christianity was one branch they grabbed to keep them from sinking, or slow them from sinking. Thus the Actions of the Emperors of this time had more to do with supporting and building their power bases against enemies real or perceived. The "Christian" church was actually involved in it's own internal struggles with Heretical teachings at this time. These "Heretical" teachings like Arianism actually controlled christianity for several decades. Dealing with such Heretics consumed most of the energy of 4th, and 5th century Christians. I think historically we call them heretics, but in reality it took about 150 years after the Romans got involved for the Christian religion to organize itself under one widely accepted set of doctrines.
It wasn't for centuries later after the coronation of Charlemagne(25 December 800 AD) where church officials began to rule over the laity rulers of Europe. Roman Emperors were themselves often deified in life or in death. They were Gods to most of their people. Constantine I who would be the first baptized Roman Emperor (baptized on his death bed) was in fact venerated by the Mithrianism(*) prior to this death. (Sol Invictus ("Unconquered Sun") was the official sun god of the later Roman Empire and a patron of soldiers. )
(*) Not to be confused with Zoroastrianism which worshiped the God Mithria in Persia. The Roman religion of the same name is mostly considered independent to the religion practiced in Persia. The Roman's brand of Mithrianism was the Romans first attempt at binding the people to the Emperor. Their brand of Mithrianism was a cult of loyalty to the Emperor and was heavily changed by the Romans from its Persian roots.
Rome's Mithrianism was based in the Roman Military and was male only.
In the Time of Charlemagne, who was crowned by the Pope, Rulers legitimacy was tied to divine right. Divine right was something a powerful church could control. Excommunicating a Ruler, who ruled by divine right or withholding the Church's approval of a ruler were powerful ways the church had to effect noblemen in the following centuries. But the 4th century was not that way.
The Inquisition, Crusades and the part of the Middle Ages referred too as the Dark Ages were some of the outcomes of this Church over State phenomena. When church leaders had the power to dictate to royals and impose their will on the state. Prior to that religious persecution was just part of the culture and less a doctrine from a central command structure. Church leaders needed Rome in the 4th and 5th centuries to make them relevant.
How did they stamp out "all other religions"? They really didn't. Long before Christians had the power over secular rulers, the "christian church" split (the great schism). The western empire fell to the barbarians and Rome catered to their new overlords breaking with Orthodoxy. Rome in the West, Orthodox Christianity in the East. When the Western Church (Catholics) was most assertive in repressing dissension the seeds of the reformation were sewn, and since the reformation Western Christianity has fractured into many branches. If you study Orthodox Christianity, Protestantism, and Roman Catholicism contemporary practitioners considered them separate religions not one religion. They fought as many wars amongst each other as they ever did amongst non Christians.
Problems: / Followup Questions
Problems: It does not explain how Christianity either provided the societal coherence the emperor was allegedly seeking
Christianity didn't. The Romans who sought to promote religion to unify the empire failed in the long run. Constantine saw christians going into the arena to be killed gruesomely singing and rejoicing. Constantine prior to becoming Augustus(Western Emperor) actively persecuted christians. He saw the Christian's behavior as a sign of selflessness and bravery which he coveted for his empire, and his legions. One essay I read in graduate school said Constantine wanted such bravery in his legions. Constantine was engrossed with what he could accomplish with a few legions of such dedicated people. Only it really never materialized. The facts were Christians of the early fourth century were pacifists, and far from being able to unify Rome, Rome spent much of the next 100 years trying to unify Christianity. There was no unity among christians in the early 4th century, Christians from different cities/continents didn't agree on details about their faith. Christianity would be consumed for the fourth century in trying to figure out which doctrine among several competing candidates(hericies) would be accepted. One doctrine would dominate for a time, and then be flipped and considered heretical, then one of the previous heretical doctrines would be accepted.. on and on it went for nearly the entire 4th century. Rome was not unified by the Christians, nor were the Christians unified quickly or very long by Rome.
or how it was able to suppress the other cults completely.
Christianity did not suppress the other cults in the fourth century. Rome did that. Why would Rome do that? Different reasons for different cults. Some pagans, like the historical Roman pagans picked the western emperor in a civil war with the eastern empire and lost. The Roman Pagans were associated with traditional Roman values in a time of dramatic change. Ultimately they bet on the wrong horse.
Some formerly permitted cults/religions lost favor with the Emperor because christianity looked more promising for their goals. In general though when the western Roman Empire really started to crumble in the 5th century. The Catholic church in the west was no longer just 10% of the population it was when when Constantine first started promoting it. They had the numbers on the Roman Pagans, and they were quick to make inroads with the Barbarians by both adopting pagan customs as well as impressing the pagans with the utility and power of their god. This is where the catholic church got incense, saint medals and various other rituals, not found in Orthodox Christianity which was not exposed to the same barbarian invasions. As Christianity made inroads and converted the barbarians invaders of Western Europe, Christianity was itself changed. That is ultimately your answer, Christianity once organized by the Romans, became more adaptable at seeking favor and courting power from the various rulers which it had to contend with in very turbulent times. That was all fifth to tenth century Christianity. As for exterminating non Christians, or Christians of other doctrines which the Catholic Church was also very active with for much of it's history, you can chalk that up to having obtained critical mass of population and just the continuation of the same kinds of xenophobic behaviors many other religions had expressed before them when in control over the same populations. Ultimately christianity would have the numbers, the organization, and political capital with the Rulers of Western Europe to exert such policies.
However, christianity did not exert control over the Emperors of Rome in the fourth century, the Roman emperors elevated, organized, supported and used the church for their own reasons. Reasons christians of the time with their constant squabbling over doctrine weren't all that helpful with.
This period in the early fourth century is also when Christianity became obsessed with Relics. The Roman Emperor would send out an expedition to the holy lands and come back with the holy grail, the cross which Jesus was crucified on, the spear of destiny, and the bones of various saints. He who owned the most holy relics must be closer to god, having collected such godly things. And that's one way in which the Roman Emperors bought influence in the church, or placed themselves above church officials. Rome was less concerned with doctrine and more concerned with the church being universally accepted, there be standards, and that it grew to be a force for unity. This is where the marriage between church and state was born. The power dynamic of that marriage would be turned on it's head in 800 AD with the coronation of Charlemagne, where ultimately the Roman Catholic Church would not only become a temporal power with the gift of Pepin(forgery), but a superpower throughout Europe. But that was all 500 years after Constantine first had the bright idea to promote religion as a seed for societal stability and unity in his empire.
Anyway when Western Rome started to withdraw from outer regions and contract in on itself (end of 4th and 5th century, Last Western Roman Emperor was Romulus Augustulus who ruled for less than one year 31 October 475 AD – September 4 476 AD). The Pagans were still there. When the Hellenistic Neoplatonist philosopher Hypatia was murdered by Christians in 415 AD, she was herself a pagan. Pagans were out of favor in the fifth century AD when Rome began contracting and ultimately fell, but they were still there.